Found this posting on ebay...

Good Day all

In view of danf's quantitative analysis results published on this forum, I could not help but smile at this recent posting on eBay for a 7199 - based replacement board (IOW, the original). The bit that got me was the "not an unknown "new improved" circuit" part:

Extract Starts here:
This is an exact replacement for the original Dynaco PC-3 board, fabricated from 0.062" FR4 material with plated-through holes with pads and solder mask on both sides pre-assembled with a complete set of audiophile grade components using silver-bearing solder. If you are serious about getting the absolute best sound from your Dynaco Stereo 70, either this board or our board kit is a must. It uses the 7199 circuit from the original, not an unknown "new improved" circuit. Its mounting holes and numbering are identical to the original Dynaco PC-3, allowing easy replacement. It is easy to install using the included installation instructions.

All the components on this board are of the highest quality brought to you at a reasonable price. I had to shop several vendors to find the quietest resistors at the best prices. All components have been carefully chosen to support the high voltages found on the ST-70 board.....

Blah blah blah
...End of Extract

-- JunkyJan

Jan,

Dynaco devised that 7199 circuit as a cost cutting measure because it allowed them to use just TWO driver tubes instead of THREE like most of the replacement driver circuits for the ST-70. When you save one tube on 350,000 amplifiers you have saved a lot of money. Remember - the design goal back in 1959 was to sell the ST-70 kit amp for $99 USD. In order to reign in some of the circuit deficiencies they had to use excessive amounts of feedback (20dB) which is pretty high by todays standards. The VTA driver board in comparison uses only 13 dB of feedback. You usually need some feeback in most push-pull designs to keep the amp stable. You can use even less feedback if you run the amp in triode mode.

If you replace your ORIGINAL ST-70 driver board with another ST-70 ORIGINAL driver board replacement you will still be stuck with the 7199 tubes. Two things about 7199 besides their IMHO mediocre sound. (smooth, mellow, non offensive but not particularly accurate) 7199 tubes are not made anymore by any company anywhere in the world. As the present stock of 7199 tubes dry up, the price on these tubes will continue to rise. Secondly, there is NO DIRECT REPLACEMENT tube for the 7199. You can rewire the socket to accept 6GH8A tubes OR purchase 7199 to 6GH8A adapters from www.dynakitparts.com to allow you to use the more readily available and cheaper 6GH8A tubes.

The demise of the ST-70 started in 1974 when the FTC mandated a new standard for comparing amplifier power output. Now all the inadequacies of the ST-70's power supply and driver board limitations came to light and Dynaco was forced to change the specs of the ST-70 from 35 WPC to 20 WPC and the power bandwith was reduced to 50 - 10,000 Hz. If you own a stock ST-70 today that is what you are looking at > really just 20 WPC and a power bandwitch of 50-10,000 Hz. See the page below from the 1974 Dynaco product brochure in which Dynaco explains the new standards by which they now have to advertise power output and distortion figures.

Bob

Hi Bob

Just to make sure that you understood the intent of my posting, I quoted the eBay entry merely for its "Entertainment Value" ;-). I have seen many threads at other forums (for instance diyTube) regarding alternatives to the original 7199 driver board because it is almost universally regarded as not acceptable by modern standards. The consensus is pretty much that the VTA board is right up there among the best.

-- JunkyJan